How can I test the mirror to see if it is functioning or not? I think it stopped working. It shares the same fuse as the power windows and everything else is working fine.

 

The last few cars I've had would dim if it was sitting in the garage where it was dark and then you opened the garage door where it was bright outside (like it was sensing the light behind it like it would if there was headlights behind you).

If you have a really bright LED flashlight or spotlight, maybe try shining it through the rear window from outside at night?

 

In previous cars I've had with dimming mirrors, there was a light sensor on the back of the mirror(closer to the windshield) and another just below the mirror. If the front sensor doesn't detect much or any light while there was light shining on the mirror side sensor, the mirror would get really dark. Easiest way to do this is to turn on the overhead light, right over the mirror at night. The mirror would go so dark it was basically not even a mirror anymore. Don't have my Mustang yet so I can't guarantee this works, but something to try. I'm pretty sure the bigger the difference between the light hitting the 2 sensors the darker the mirror gets.